So I ported the original patch mentioned to the latest Debian version. Well at least the bits that matter to me. See below.

In doing so I realized that it does quite work how I imagined it would; instead of generating a dynamic page for a particular day with a list of links it actually fills in the calendar with the details of the posts (making for some ugly formatting).

I'm hoping the tag generation pages will give me a clue how to alter this into what I want.

To be clear, this patch creates a yyyy/mm/dd file for each day, listing the posts for that day, so the calendar can link to it rather than a random single post.

While a valid solution certainly, that's a lot of added pages! Especially a high overhead for such a minor UI point as this.

Surely something interesting could be done with javascript or some other form of UI to make clicking on a day in a calendar that has multiple posts present a list of them? That would have essentially no overhead, since the calendar plugin already has a list of the posts made on a given day.

Ikiwiki already does something similar to deal with the case where a page has a great many backlinks.. It makes a UI element that, if hovered over, pops up a display of all the rest. This is done quite simply in the page.tmpl
using the popup and balloon CSS classes. Calendar could also use this.

if there is a single entry in one day, does not change anything (compared to the previous version of the calendar plugin);

if there are several entries, when mouse passes over the day, displays a popup listing all the entries of that day.

That's all. No new pages for each day, takes as little space as it took before, and only a few lines more in the source.

The only thing I am not totally happy with is the CSS. We have to say that the text is aligned on the left (otherwise, it is aligned on the right, as is each day of the calendar), but I do not know which place is the more sensible to put that line of CSS in.